Biomphalaria and Bulinus Spp
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Acta Tropica 132 (2014) 64–74 Contents lists available at ScienceDirect Acta Tropica jo urnal homepage: www.elsevier.com/locate/actatropica Regulation of laboratory populations of snails (Biomphalaria and Bulinus spp.) by river prawns, Macrobrachium spp. (Decapoda, Palaemonidae): Implications for control of schistosomiasis a,∗,1 b a Susanne H. Sokolow , Kevin D. Lafferty , Armand M. Kuris a Ecology Evolution and Marine Biology Department, University of California Santa Barbara, Santa Barbara, CA, 93106, USA b Western Ecological Research Center, US Geological Survey, c/o Marine Science Institute, University of California, Santa Barbara, CA, 93106, USA a r a t i c l e i n f o b s t r a c t Article history: Human schistosomiasis is a common parasitic disease endemic in many tropical and subtropical Received 29 October 2013 countries. One barrier to achieving long-term control of this disease has been re-infection of treated Received in revised form patients when they swim, bathe, or wade in surface fresh water infested with snails that harbor and 13 December 2013 release larval parasites. Because some snail species are obligate intermediate hosts of schistosome para- Accepted 19 December 2013 sites, removing snails may reduce parasitic larvae in the water, reducing re-infection risk. Here, we Available online 31 December 2013 evaluate the potential for snail control by predatory freshwater prawns, Macrobrachium rosenbergii and M. vollenhovenii, native to Asia and Africa, respectively. Both prawn species are high value, protein-rich Keywords: Predation human food commodities, suggesting their cultivation may be beneficial in resource-poor settings where few other disease control options exist. In a series of predation trials in laboratory aquaria, we found both Biological control Functional response species to be voracious predators of schistosome-susceptible snails, hatchlings, and eggs, even in the pres- Predator ence of alternative food, with sustained average consumption rates of 12% of their body weight per day. Schistosoma mansoni Prawns showed a weak preference for Bulinus truncatus over Biomphalaria glabrata snails. Consumption Schistosoma haematobium rates were highly predictable based on the ratio of prawn: snail body mass, suggesting satiation-limited predation. Even the smallest prawns tested (0.5–2 g) caused snail recruitment failure, despite high snail fecundity. With the World Health Organization turning attention toward schistosomiasis elimination, native prawn cultivation may be a viable snail control strategy that offers a win–win for public health and economic development. © 2013 Elsevier B.V. All rights reserved. 1. Introduction surface fresh water. A few weeks after infection, snails release larval schistosomes into the water where they can infect people, comple- Human schistosomiasis is a common parasitic disease of ting the lifecycle. Breaking this lifecycle through snail control was humans that is relatively easy to treat, but hard to control. Today, an approach used extensively for schistosomiasis control prior to public health campaigns in endemic regions in the tropics and the advent of the drug praziquantel. Snail control aims to reduce subtropics focus on mass drug administration using the oral drug, the number of parasitic larvae in the water, effectively reduc- praziquantel. While praziquantel has a high cure rate, re-exposure ing reinfection prevalence and intensity. Traditional approaches to infected snails in the environment leads to rapid reinfection of to snail control using molluscicide application and habitat mod- treated patients in endemic areas (Fenwick et al., 2006; Fenwick ification can be expensive to implement and hard to maintain and Webster, 2006; King et al., 2006; Tchuem Tchuente et al., over the long-term. In some areas, researchers have documented 2013; Webster et al., 2013). In addition to drug treatment, a a negative relationship between natural aquatic snail predators complementary approach is to control the populations of snails and the density of schistosome-susceptible snails, such as occurs that serve as intermediate hosts. Snails are infected when para- in regions of Lake Malawi, where overfishing pressure may have site eggs, released in human urine or feces, are washed into the caused snail populations to increase after predators were removed (Evers et al., 2006; Madsen and Stauffer, 2011). This suggests that in some ecological situations, control of snails through predator introductions could offer an effective snail control strategy that ∗ Corresponding author at: Stanford University, Hopkins Marine Station, 20 is affordable and sustainable and that may complement ongo- Oceanview Blvd, Pacific Grove, CA 93950, USA. Tel.: +1 831 5153205.. ing drug distribution campaigns. Here, we evaluate the potential E-mail address: [email protected] (S.H. Sokolow). 1 for snail control by predatory freshwater prawns in the genus Present affiliation: Stanford University, Hopkins Marine Station, Pacific Grove, CA 93950, USA. Macrobrachium. 0001-706X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.actatropica.2013.12.013 S.H. Sokolow et al. / Acta Tropica 132 (2014) 64–74 65 Macrobrachium vollenhovenii, is a freshwater prawn native to characterize the functional response of prawns when offered vary- rivers and streams throughout West Africa. We hypothesized that ing sizes and densities of snails, as would be found in natural M. vollenhovenii would be an effective snail predator because this populations. Finally, we aimed to synthesize this information to species shares similar habitats with medically important snails, guide the development of a new strategy for sustainable schistoso- grows to a large size, and because the congener Macrobrachium miasis control and elimination through restoration or stocking of rosenbergii (an Asian species) consumes snails as a preferred food river prawns in schistosomiasis-endemic areas, especially through- (Lee et al., 1982; Roberts and Kuris, 1990). In Senegal, the con- out Africa where the highest schistosomiasis transmission rates are struction of the Diama Dam to block tidal influence in the Lower found today. Senegal River created a large freshwater irrigation system that offered abundant habitat to medically important snails, hosts of 2. Methods both Schistosoma mansoni and Schistosoma haematobium. This was associated with a severe and persistent outbreak of schistosomia- 2.1. Animals sis involving both species, with increased snail abundance resulting from expanded low-flow, freshwater snail habitat after dam com- Uninfected, laboratory-reared Biomphalaria glabrata, strain pletion as well as probable immigration of infected agricultural NMRI, and Bulinus truncatus, subspp. truncatus, were supplied by workers to the region (Southgate, 1997; Sow et al., 2002). The the Schistosomiasis Resources Center (BEI Resources, Manassas, Diama Dam also presumably blocked the migration of native M. VA). Laboratory-reared M. rosenbergii juvenile prawns were sup- vollenhovenii prawns to their estuarine breeding grounds. Although plied by the Aquaculture Department at Kentucky State University other impacts of the dam were not investigated, local fishermen and delivered by airfreight to the University of California Santa reported that prawns were once common, but declined sharply Barbara. Captive populations of M. vollenhovenii prawns were not after dam construction. Since prawns have been shown in labora- available, so wild-caught prawns were collected from the Lobe tory studies to be voracious and effective predators of Biomphalaria River, Cameroon (Gulf Aquatics-Cameroon, Duoung, Cameroon) glabrata (Lee et al., 1982; Roberts and Kuris, 1990), we speculate and delivered by air freight in December 2011 to Kentucky that a loss of prawns above Diama Dam may have contributed to State University’s Aquaculture Department. At Kentucky State, the increase in snail intermediate hosts in the Lower Senagal River the prawns were captively bred and the first generation juve- Basin, and therefore, an increase in schistosomiasis transmission. If nile prawns were delivered by airfreight to UC Santa Barbara in so, restoration of M. vollenhovenii to the Senegal River might con- June 2012. Prawns and snails were housed in closed, recirculat- tribute to schistosomiasis control in that region or other similar ing freshwater tanks at UC Santa Barbara’s Marine Biotechnology regions of the world where schistosomiasis has increased after dam Laboratory. The tank system had both mechanical and biological construction (Steinmann et al., 2006). filtration, continuous aeration, and 20% weekly water exchanges Roberts and Kuris (1990) published a series of laboratory tri- using conditioned tap water. Between experiments, prawns and als that built on earlier work (Lee et al., 1982) demonstrating that snails were housed in holding tanks: a 400 L common holding tank M. rosenbergii – the most commonly aquacultured species of fresh- for prawns, and four 25 L holding tanks for snails. Both during water prawn worldwide – can consume B. glabrata snails. Roberts and between experiments, prawns were fed a commercial shrimp and Kuris concluded that prawn cultivation may offer a valuable crumble diet with 40% protein content (Rangen Corporation, Buhl, complementary strategy for schistosomiasis control activities. Yet, ID) at a rate of 3–5% body weight per day, 5 days per week, and to date, biological control using crustacean snail-predators has not snails were